Process for the preparation of a catalyst based on cobalt and scandium

ABSTRACT

A process for the preparation of a catalyst comprising sequential deposition of cobalt or a cobalt compound, and scandium or a scandium compound, on an inert carrier oxide, which catalyst can be used in the conversion of synthesis gas according to the Fischer-Tropsch process.

This application is a 371 of PCT/EP98103873 filed on Jun. 20, 1998.

The present invention relates to a process for the preparation of acatalyst and its use in the conversion of synthesis gas according to theFischer-Tropsch process.

More specifically, the present invention relates to a process for thepreparation of a catalyst comprising Co and Sc supported on an inertcarrier.

The selection of cobalt is due to the fact that this favours theformation of paraffins with a high molecular weight preventing theformation of branched products, olefins and compounds containing oxygen.

The use of catalysts based on cobalt goes back to the first works ofFischer in 1932 (H. H. Storch, N. Golumbic, R. B. Anderson, "The FischerTropsch and Related Synthesis", John Wiley & son, Inc., New York, 1951,pages 345-367) which developed the Co/ThO₂ /Mg0/kieselguhr system.

The evolution of these systems subsequently led to the idenfitication ofvarious promoters to be coupled with cobalt in order to increase theselectivity to hydrocarbons with a high moleuclar weight, this mainly inthe last twenty years. In fact, the increase in price of crude oil inthe 70s' provided the incentive for exploring other ways of producingliquid fuels and chemicals.

U.S. Pat. No. 4,088,671 describes a catalyst for Fischer-Tropsch havingcobalt and ruthenium as active ingredients, the former being present ina larger quantity with respect to the latter.

WO93/05000 describes a catalyst essentially consisting of cobalt (1-50%by weight), scandium (0.01-25% by weight) and an appropriate carrierselected from alumina, silica, silica-alumina, kieselguhr, preparedaccording to the usual preparation techniques among which impregnationof the carrier with aqueous solutions of the corresponding salts. Theabove catalyst, particularly efficient in the conversion of synthesisgas to give a hydrocarbon product with a high content of paraffins, hasthe disadvantage however of requiring high reaction temperatures andproducing large quantities of methane.

A process has been found for the preparation of a catalyst, supported onan inert material, essentially consisting of a larger quantity of cobaltand a smaller quantity of scandium, which overcomes the drawbacksmentioned above. With this process, in fact, it is possible to obtain asupported catalyst based on cobalt and scandium which allows highconversions of CO to paraffins with a high molecular weight with lowselectivities to methane, operating at lower temperatures than thosedescribed in WO 93/05000.

In accordance with this, the present invention relates to a process forthe preparation of a catalyst comprising an inert carrier selected fromat least one oxide of at least one element selected from Si, Ti, Al, Zr,Zn, Mg, Sn, preferably silicon, in the form of elements or oxides, alarger quantity of cobalt and smaller quantities of scandium,characterized in that it comprises at least the following steps:

1) production of a first catalytic precursor (A) containing cobalt andat least part of the inert carrier, by the deposition of cobalt on theinert carrier; subsequent calcination, reduction and passivation of theinert carrier containing cobalt;

2) production of the final catalyst by the deposition of scandium on thecatalytic precursor (A); subsequent calcination, reduction andpassivation of the inert carrier containing cobalt and scandium.

A further object of the present invention relates to the catalyst whichcan be obtained with the above process.

In the process of the present invention, step (1) consists in an initialdeposition of cobalt on the inert carrier. This deposition, like that ofthe second element in step (2), can be carried out according to varioustechniques known to experts in the field, for example exchange,impregnation, dry impregnation (also called of the incipientimbibition), precipitation, gelation and mechanical mixing.

In the preferred embodiment, the deposition of cobalt in step (1) iscarried out by the dry impregnation technique. According to this methodthe material to be impregnated is put in contact with a volume ofsolution more or less equal to the pore volume.

In step (1) it is preferable to use aqueous solutions of cobalt salts.Any kind of cobalt salts can be used, for example halides, nitrate,acetate, oxalate, the complex formed with lactic acid and lactates, thecomplex formed with tartaric acid and tartrates, the complex formed withanother polyacid or hydroxyacid and the relative salts, the complexformed with acetylacetonates.

After depositing the desired quantity of cobalt salt, preferably cobaltnitrate, onto the inert carrier, a calcination step is carried out,followed by a reduction and passivation step. Optionally, before thecalcination, the impregnated carrier is subjected to drying to eliminatemost of the water. This drying can be carried out first at temperaturesof between 10 and 30° C. and subsequently at temperatures of between 100and 120° C., preferably in a stream of gas.

In step (1) the calcination is carried out at a temperature of between300° C. and 500° C., preferably between 350° C. and 450° C., in anenvironment of air to eliminate all the organic residues.

The product thus calcined is then subjected to a reduction step in anenvironment essentially consisting of hydrogen, at a temperature ofbetween 300° C. and 500° C., more preferably from 350° C. to 450° C. Itis preferable to gradually bring the substrate to be calcined to thistemperature, for example with a heating rate of between 3 and 20°C./minute. The reduction step is usually completed at the abovetemperature in a time of from 10 to 20 hours and with a stream of H₂ ofbetween 1 and 3 liters/hour per gram of catalyst.

At the end of the reduction step, a passivation step is carried out inthe presence of oxygen diluted with an inert gas, usually nitrogen,preferably carried out at a temperature of between 10° C. and 80° C.Using, for example, nitrogen containing 1-2% of O₂ (stream of 2liters/hour), the above step can have a duration of from 1 to 5 hours at25° C.

It is evident that at the end of the reduction (and obviously before thepassivation), the sample must be cooled.

The second and last step of the process of the present inventionconsists in depositing the desired quantity of scandium onto theprecursor (A), obtained at the end of the first step.

In one embodiment, a scandium nitrate is used, dissolved in solventselected from acetone, lower alcohols, water and the relative mixtures.In step (2), the preferred technique is the wet impregnation, whichessentially consists in immerging the precursor (A) into the solution ofScandium and eliminating the solvent with a slow evaporation systemunder vacuum.

As for step 1, after depositing the scandium, there is a calcinationstep followed by a reduction and subsequently a passivation step. Inthis case however, it is preferable to carry out the calcination step ata slightly lower temperature with respect to the calcination temperatureof step 1, i.e. from 200° C. to 400° C., preferably from 205° C. to 350°C. The reduction and passivation on the other hand are carried out underthe same temperature conditions as step 1.

The catalytic composition which can be obtained with the process of thepresent invention contains a larger quantity of cobalt (in metal form orin the form of a derivative) and a smaller quantity of scandium, as ametal or in the form of a derivative. Both the cobalt and the scandiumare dispersed on the carrier and, when present in the form of aderivative, the oxide form is preferred.

As already specified, the carrier consists of at least one oxideselected from at least one of the following elements: Si, Ti, Al, Zr,Zn, Mg, Sn. In the preferred embodiment the inert carrier is silica.

The content of the above elements in the final catalyst, expressed asmetal content and defined as weight percentage with respect to theweight of the catalyst, is from 1 to 50%, preferably from 5 to 35% forthe Cobalt, whereas it is from 0.05 to 5%, preferably from 0.1 to 3% forthe Scandium.

As already mentioned, the present invention also relates to a processfor the preparation of hydrocarbons from synthesis-gas (Fischer-Tropschprocess) in the presence of the catalytic system described above.

The present invention relates to a catalytic composition which allowsthe mixture of CO and H₂, known as synthesis gas, to be converted intoessentially saturated and linear hydrocarbons having a C₂₅ ⁺ content ofbetween 25 and 29% by weight for hourly volumetric flow-rate values(GHSV=Gas Hourly Space Velocity) of between 500 and 1500 h⁻¹.

The conditions for using these catalysts are, in turn, those alreadyknown in the art for the embodiment of the Fischer-Tropsch synthesis.

The conversion of the synthesis gas to hydrocarbons takes place at apressure normally between 0.1 and 15 MPa, preferably from 1 to 10 MPa,at a temperature generally within the range of 150° C. to 350° C.,preferably from 170° C. to 300° C. A lowering of the reactiontemperature generally causes an increase in the selectivity tohydrocarbon products with a high molecular weight, but an inevitabledecrease in the conversion of the syngas (CO conversion). There aretherefore selectivity and conversion limits governed by economicconsiderations which impose definite fields of practice under thereactions conditions to be used. These limits can be overcome by the useof particularly selective catalytic systems with respect to thehydrocarbon fractions with a high molecular weight (for example C₂₅ ⁺).

The hourly volumetric velocity of the reagent gas is generally from 100to 20000, preferably from 400 to 5000, volumes of synthesis gas pervolume of catalyst and per hour; The ratio H₂ /CO in the synthesis gasis generally from 1:2 to 5:1, preferably from 1.2:1 to 2.5:1.

The catalyst can be used in the form of fine powder (about 10-700 mm) orin the form of particles having an equivalent diameter of from 0.7 to 10mm, respectively in the presence of a liquid phase (under the operatingconditions) and a gaseous phase, or a gaseous phase. The liquid phasecan consist of at least one hydrocarbon having at least 5, preferably atleast 10, carbon atoms per molecule. In the preferred embodiment, theliquid phase essentially consists of the same reaction product.

Just to give an example, the catalysts of the present invention can beused in a fixed-bed reactor, fed in continuous with a mixture of Co andH₂ and operating under the following conditions:

    ______________________________________                                        reaction temperature 200-220° C.                                       reaction pressure    20 bars                                                  space velocity       500-1500 h.sup.-1                                        H.sub.2 /CO mixture  2/1                                                      ______________________________________                                    

Following these conditions, the catalysts prepared in examples 1 to 5were evaluated and their compositions are summarized in table 1. Theresults of the reactivity tests are indicated in table 2.

EXAMPLE 1

Catalyst A (Reference)

Silica is used, having a surface area of 300 m^(2/) g, a specific porevolume of 1.3 cm³ /g, a particle diameter of 20 mm, a specific weight of0.388 g/cc.

The above silica is dry impregnated with a nitric solution ofCo(NO₃)₂.6H₂ O in such quantities as to obtain a percentage of Co equalto 15% by weight referring to the total. The silica thus impregnated isdried at 120° C. for 16 hours and calcined at 400° C. in air for 4hours, then treated in a stream of H₂ at a space velocity (GHSV) of 1000h⁻¹, in a tubular reactor at 400° C. for 16 hours. The sample thusreduced is passivated in a mixture of (1%) O₂ /(99%) N₂ with GHSV at1000 h⁻¹ for 2 hours at room temperature. (Catalyst A: Co/SiO₂ ; 15%Co).

EXAMPLE 2

Catalyst B

For the preparation of catalyst B, a solution of Sc(NO₃)₂ 10⁻³ M inacetone is added to 50 g of catalyst A, in such a volume as to obtain afinal weight percentage of Sc equal to 0.1%.

The suspension thus obtained is left under stirring for two hours and isthen dried under vacuum at 40° C. The sample is calcined at 300° C. for4 hours in air, reduced at 400° C. in H₂ for 16 hours with a GHSV of1000 h-1 at room temperature and passivated in (1%)O₂ /(99%)N₂ with aGHSV of 1000 h⁻¹ for 2 hours at room temperature. (Catalyst B:Co/Sc/SiO₂ 15% Co, 0.1% Sc).

EXAMPLE 3

Catalyst C

The preparation of catalyst C differs from that described in example 2in the use of a solution of Sc(NO₃)₂ 10⁻³ M in acetone in such a volumeas to obtain a final weight percentage of scandium equal to 0.4%.(Catalyst C: Co/Sc/SiO₂ 15% Co, 0.4% Sc).

EXAMPLE 4

Catalyst D

The preparation of catalyst D differs from that described in example 2in the use of a solution of Sc(NO₃)₂ 10⁻³ M in acetone in such a volumeas to obtain a final weight percentage of scandium equal to 0.2%.(Catalyst D: Co/Sc/SiO₂ 15% Co, 0.2% Sc).

COMPARATIVE EXAMPLE 5

Catalyst E

This catalyst is prepared according to what is described in example 18of WO 93/05000.

42.4 g of silica (surface area=540 m² /g; average pore volume=0.9 cc/g;specific weight =0.42 g/cc) are used as carrier for the catalyst.

An aqueous solution of cobalt nitrate and scandium nitrate is thenprepared by dissolving 20.83 g of Co(NO₃)₂.6H₂ O and 2 g of Sc(NO₃)₂.5H₂O in water.

Following the impregnation technique, the carrier is impregnated withthis solution and the solvent is eliminated with an evaporation systemunder vacuum (rotating evaporator), dried and calcined at 500° C. for 4hours in muffle.

A product is obtained having a content of Co of 7.5% w/w and Sc of 0.5%w/w.

The characteristics of the catalysts thus prepared are indicated intable 1.

                  TABLE 1                                                         ______________________________________                                        Example    Catalyst                                                                              % Co       % Sc Carrier                                    ______________________________________                                        Comp. 1    A       15         --   SiO.sub.2                                  2          B       15         0.1  SiO.sub.2                                  3          C       15         0.4  SiO.sub.2                                  4          D       15         0.2  SiO.sub.2                                  Comp. 5    E       7.5        0.5  SiO.sub.2                                  ______________________________________                                    

The catalysts thus prepared were tested for the Fischer-Tropsch reactionunder the conditions specified above.

The results are shown in table 2.

                  TABLE 2                                                         ______________________________________                                               A     B       C       D(1)  D(2)  E                                    ______________________________________                                        T(° C.)                                                                         210     205     210   208   210   218                                Conv. CO (%)                                                                           55.12   82.86   78.95 57.61 77.64 32.5                               Prod. C.sub.2 .sup.+                                                                   87.41   122.41  161.21                                                                              272.61                                                                              351.74                                                                              187.24                             (g/Kg/h)                                                                      Co-T-Y   3.51    4.02    6.38  9.31  12.74 9.81                               Sel. C.sub.1 -C.sub.4                                                                  20.57   20.95   19.37 17.25 22.3  24.05                              Sel. C.sub.10 -C.sub.24                                                                43.58   41.61   44.17 66.28 54.74 49.05                              Sel. C.sub.25 .sup.+                                                                   23.77   23.55   24.96 7.83  5.52  4.19                               Sel. C.sub.5 .sup.+                                                                    81.17   80.36   81.94 82.75 77.7  75.95                              Sel. CO.sub.2                                                                          0.8     2.99    1.49  0.04  0.14  0.42                               GHSV     500     500     625   1.500 1.500 1.500                              ______________________________________                                    

From a comparison between the catalyst without the second element(comparative catalyst A) and the catalyst of the present invention (B),it is evident that the system with two elements is more active operatingat lower temperatures, with a higher selectivity value to C₂₅ ⁺ (28.55wt %), with decisively better hourly weight productivities tohydrocarbons with more than two carbon atoms (Prod. C₂ ⁺) and yields toproducts containing carbon (Cobalt-Time-Yield=CoTY). More specifically,the CoTY is a useful parameter for comparing the catalysts with adifferent content of cobalt as it normalizes the yield to productscontaining carbon (hydrocarbons and CO₂) in relation to the moles of Coavailable.

CoTY=moles conv. CO/moles Co tot/h

The activities of the catalysts of the present invention (B and C) arealmost the same even if an increase in the content of Sc causes anincrease in the selectivity to hydrocarbons higher than C₅ as can beseen in Table 2. In example C, the lower selectivity to heavy C₂₅ ⁺hydrocarbons is influenced by the higher volumetric flow-rate of thereagent gas (GHSV=625).

On comparing the comparative catalyst E with the catalyst of the presentinvention D, having an intermediate content of Sc between B and C, itcan be observed how with the same GHSV, catalyst D (2) gives highervalues of CoTY, although operating at lower temperatures (210° C.), andselectivity to heavy hydrocarbons (C₂₅ ⁺).

A comparison between catalyst D and comparative catalyst E also showshow higher temperatures cause the formation of lighter products. Infact, with the same CoTY, the lower temperature at which catalyst D(1)operates, allows lower C₁ -C₄ selectivities and favours the selectivityto both C₂₅ ⁺ and C₅ ⁺.

EXAMPLE 6

Catalyst F

The silica used in example 1 is dry impregnated with a nitric solutionof Co(NO₃)₂.6H₂ O in such quantities as to obtain a percentage of Coequal to 7.5% by weight referring to the total.

The silica thus impregnated is dried at 120° C. for 16 hours andcalcined at 400° C. in air for 4 hours, then treated in a stream ofhydrogen at a space velocity (GHSEI) of 1000 h⁻¹, in a tubular reactorat 400° C. for 16 hours. The sample thus reduced is passivated in amixture (1%)O₂ /(99%)N₂ with a GHSV at 1000 h⁻¹ for 2 hours at roomtemperature.

A 10⁻³ solution in acetone of Sc(No₃)₂ is added to this precursor, insuch a volume as to obtain a final weight percentage of Scandium equalto 0.5%.

The suspension thus obtained is left under stirring for two hours andthen dried under vacuum at 40° C. The sample is calcined at 300° C. for4 hours in air, reduced at 400° C. in H₂ for 16 hours with a GHSV of1000 h⁻¹ at room temperature and then passivated in (1%)O² / (99%)N₂with a GHSV of 1000 h⁻¹ for 2 hours at room temperature.

The catalyst thus prepared (Co=7.5%, Sc=0.5) is tested for theFischer-Tropsch reaction according to the above procedure. The resultsare shown in table 3, which indicates by comparison, the data obtainedfor the catalyst (catalyst G) described in example 18 of WO 93/05000having the same composition (Co=7.5%; Sc=0.5%).

                  TABLE 3                                                         ______________________________________                                                        Cat. F                                                                              Comp. Cat. G                                            ______________________________________                                        Temperature (° C.)                                                                       240     240                                                 Conversion CO (%) 80.11   68                                                  Production C.sub.2 .sup.+  (g/Kg/h)                                                             266.1   153                                                 Co-T-Y            12.64   --                                                  Selectivity C.sub.2 .sup.+ (w/w %)                                                              88.45   60                                                  Selectivity CH.sub.4 (C %)                                                                      11.55   35                                                  Selectivity C.sub.1 -C.sub.4 (w/w %)                                                            18.52   --                                                  Selectivity C.sub.10 -C.sub.24 (w/w %)                                                          49.52   --                                                  Selectivity C.sub.25 .sup.+  (w/w %)                                                            23.33   --                                                  Selectivity C.sub.5 .sup.+  (w/w %)                                                             81.48   --                                                  Selectivity CO.sub.2 (w/w %)                                                                    0.66    5                                                   GHSV              1.000   1.000                                               ______________________________________                                    

The data of table 3 show how the two catalysts, one of the presentinvention and the other of the prior art, give completely differentperformances even though they have the same composition (7.5% of Co and0.5% of Sc for both).

In fact, catalyst F of the present invention gives, with respect to thecatalyst of the prior art, considerably higher conversions of CO andproductivity to C₂ ⁺, as well as much lower selectivity to methane.

What is claimed is:
 1. A process for the preparation of a catalystcomprising an inert carrier selected from at least one oxide of at leastone element selected from the group consisting of Si, Ti, Al, Zr, Zn,Mg, and Sn, in the form of elements or oxides, (cobalt, and X scandiumin an amount less than the amount of cobalt, which comprises at leastthe following steps:1) production of a first catalytic precursor (A)containing cobalt and at least part of the inert carrier, by thedeposition of cobalt on the inert carrier; subsequent calcination,reduction and passivation of the inert carrier containing cobalt; 2)production of the final catalyst by the deposition of scandium on thecatalytic precursor (A); subsequent calcination, reduction andpassivation of the inert carrier containing cobalt and scandium.
 2. Theprocess according to claim 1, wherein the inert carrier is silica. 3.The process according to claim 1, wherein the catalyst has a content ofcobalt of from 1 to 50% by weight and a content of scandium of from 0.05to 5% by weight.
 4. The process according to claim 3, wherein the cobaltis contained in a quantity of from 5 to 35% and the scandium in aquantity of from 0.1 to 3% by weight.
 5. The process according to claim1, wherein in step (1) the calcination is carried out at a temperatureof from 350° C. to 450° C., whereas in step (2) the calcination iscarried out at a temperature of from 250° C. to 350° C.
 6. The catalystwhich can be obtained according to claim
 1. 7. A process for thesynthesis of essentially linear and saturated hydrocarbons starting fromsynthesis gas comprising CO and H₂, which comprises reacting thesynthesis gas with a catalyst prepared according to claim 1, at apressure of from 0.1 to 15 MPa, the temperature being from 150° C. to350° C., at an hourly volumetric velocity of from 100 to 20000 volumesof synthesis gas per volume of catalyst per hour, the molar ratio H₂ /COin the synthesis gas being from 1:2 to 5:1.
 8. The process according toclaim 7, wherein the pressure is from 1 to 10 MPa, the temperature from170° C. to 300° C., the hourly volumetric velocity from 400 to 5000volumes of synthesis gas per volume of catalyst per hour, the ratio H₂/CO in the synthesis gas is from about 1.2:1 to about 2.5:1.
 9. Thecatalyst which can be obtained according to claim
 2. 10. The catalystwhich can be obtained according to claim
 3. 11. The catalyst which canbe obtained according to claim
 4. 12. The catalyst which can be obtainedaccording to claim
 5. 13. A process for the synthesis of essentiallylinear and saturated hydrocarbons starting from synthesis gas comprisingCO and H₂, which comprises reacting the synthesis gas with a catalystprepared according to claim 2, at a pressure of from 0.1 to 15 MPa, thetemperature being from 150° C. to 350° C., at an hourly volumetricvelocity of from 100 to 20000 volumes of synthesis gas per volume ofcatalyst per hour, the molar ratio H₂ /CO in the synthesis gas beingfrom 1:2 to 5:1.
 14. A process for the synthesis of essentially linearand saturated hydrocarbons starting from synthesis gas comprising CO andH₂, which comprises reacting the synthesis gas with a catalyst preparedaccording to claim 3, at a pressure of from 0.1 to 15 MPa, thetemperature being from 150° C. to 350° C., at an hourly volumetricvelocity of from 100 to 20000 volumes of synthesis gas per volume ofcatalyst per hour, the molar ratio H₂ /CO in the synthesis gas beingfrom 1:2 to 5:1.
 15. A process for the synthesis of essentially linearand saturated hydrocarbons starting from synthesis gas comprising CO andH₂, which comprises reacting the synthesis gas with a catalyst preparedaccording to claim 4, at a pressure of from 0.1 to 15 MPa, thetemperature being from 150° C. to 350° C., at an hourly volumetricvelocity of from 100 to 20000 volumes of synthesis gas per volume ofcatalyst per hour, the molar ratio H₂ /CO in the synthesis gas beingfrom 1:2 to 5:1.
 16. A process for the synthesis of essentially linearand saturated hydrocarbons starting from synthesis gas comprising CO andH₂, which comprises reacting the synthesis gas with a catalyst preparedaccording to claim 5, at a pressure of from 0.1 to 15 MPa, thetemperature being from 150° C. to 350° C., at an hourly volumetricvelocity of from 100 to 20000 volumes of synthesis gas per volume ofcatalyst per hour, the molar ratio H₂ /CO in the synthesis gas beingfrom 1:2 to 5:1.